There has been practically used a warning system which detects whether a person is sit on a seat in a vehicle, such as an automobile and then alerts if the person does not couple a seat belt. The warning system generally gives off an alert when it detects the sitting of the person and simultaneously detects not coupling the seat belt. The apparatus generally comprises a sitting sensor which detects whether a person is sitting on a seat and a sensor which detects not coupling the seat belt with a buckle although the person is seated, which gives off an alert when the uncoupling of the seat belt, is detected. The sitting sensor necessitates high durability because it must detect a person sitting down many times. It is also necessary that, when a person is seated, the person does not feel the sensation of any foreign object in the seat.
JP 2012-108113 A (Patent Document 1) discloses a sitting sensor equipped in a seat, detecting the sitting of a person, which comprises electrodes facing with each other in a cushion material and detects an electric contact of the electrodes. This sensor employs an electrode and should equip wiring. The wiring can be disconnected by receiving a large displacement and gives some problems in durability. In addition, the electrode is generally made of metallic substance which may create a sensation of a foreign object. Even if the electrode is not metallic, the feeling of a foreign object would easily generate based on the other substances.
JP 2011-255743 A (Patent Document 2) discloses an electrostatic capacitance-type sitting sensor which comprises sensor electrodes facing with each other, between which dielectric substance is inserted, and an electrostatic capacitance-type sensor that measures an electrostatic capacity between the electrodes. This sensor also employs electrodes and should equip wiring, which gives rise to durability problems as same with Patent Document 1. It, is also difficult to prevent a sensation of a foreign object.
JP 2007-212196 A (Patent Document 3) discloses a load detection device for a vehicle seat, which comprises a magnetism generator equipped with a displaceable flexible element and a magnetic sensor, equipped with a fixing element of a flame, having a magnetic impedance element that detects a magnetic field generated by the magnetism generator. Since the magnetism generator includes a magnet having a specified size in this device, it is quite difficult to dispose the magnetism generator near a surface of a cushion material without any foreign object sensation. In order to avoid the foreign object sensation, it is considered that the magnetism generator is disposed inside the cushion material, but this leads to the deterioration of detection accuracy.
JP 2006-014756 A (Patent Document 4 discloses a biosignal detection device which comprises a permanent magnet and a magnetic sensor. Since the device also employs the permanent magnet which would give a foreign object sensation, it is difficult to place the device near a surface of the cushion material. The displacement of the device inside the cushion material leads to the deterioration of detection accuracy.